/*
 * Copyright (c) 2003, 2006, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */



package java.util;

import java.util.NoSuchElementException;

/**
 * Linked list implementation of the <tt>List</tt> interface.  Implements all
 * optional list operations, and permits all elements (including
 * <tt>null</tt>).  In addition to implementing the <tt>List</tt> interface,
 * the <tt>LinkedList</tt> class provides uniformly named methods to
 * <tt>get</tt>, <tt>remove</tt> and <tt>insert</tt> an element at the
 * beginning and end of the list.  These operations allow linked lists to be
 * used as a stack, queue, or double-ended queue (deque).<p>
 *
 * The class implements the <tt>Queue</tt> interface, providing
 * first-in-first-out queue operations for <tt>add</tt>,
 * <tt>poll</tt>, etc. Other stack and deque operations could be
 * easily recast in terms of the standard list operations.  They're
 * included here primarily for convenience, though they may run
 * slightly faster than the equivalent List operations.<p>
 *
 * All of the operations perform as could be expected for a doubly-linked
 * list.  Operations that index into the list will traverse the list from
 * the beginning or the end, whichever is closer to the specified index.<p>
 *
 * <b>Note that this implementation is not synchronized.</b> If multiple
 * threads access a list concurrently, and at least one of the threads
 * modifies the list structurally, it <i>must</i> be synchronized
 * externally.  (A structural modification is any operation that adds or
 * deletes one or more elements; merely setting the value of an element is not
 * a structural modification.)  This is typically accomplished by
 * synchronizing on some object that naturally encapsulates the list.  If no
 * such object exists, the list should be "wrapped" using the
 * Collections.synchronizedList method.  This is best done at creation time,
 * to prevent accidental unsynchronized access to the list: <pre>
 *     List list = Collections.synchronizedList(new LinkedList(...));
 * </pre><p>
 *
 * The iterators returned by the this class's <tt>iterator</tt> and
 * <tt>listIterator</tt> methods are <i>fail-fast</i>: if the list is
 * structurally modified at any time after the iterator is created, in any way
 * except through the Iterator's own <tt>remove</tt> or <tt>add</tt> methods,
 * the iterator will throw a <tt>ConcurrentModificationException</tt>.  Thus,
 * in the face of concurrent modification, the iterator fails quickly and
 * cleanly, rather than risking arbitrary, non-deterministic behavior at an
 * undetermined time in the future.
 *
 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification.  Fail-fast iterators
 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness:   <i>the fail-fast behavior of iterators
 * should be used only to detect bugs.</i><p>
 *
 * This class is a member of the
 * <a href="{@docRoot}/../guide/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @author  Josh Bloch
 * @version 1.61, 02/19/04
 * @see	    List
 * @see	    ArrayList
 * @see	    Vector
 * @see	    Collections#synchronizedList(List)
 * @since 1.2
 * @param <E> the type of elements held in this collection
 */

public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Queue<E>, Cloneable
{
    private transient Entry<E> header = new Entry<E>(null, null, null);
    private transient int size = 0;

    /**
     * Constructs an empty list.
     */
    public LinkedList() {
        header.next = header.previous = header;
    }

    /**
     * Constructs a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *
     * @param  c the collection whose elements are to be placed into this list.
     * @throws NullPointerException if the specified collection is null.
     */
     public LinkedList(Collection<? extends E> c) {
	 this();
	 addAll(c);
     }

    /**
     * Returns the first element in this list.
     *
     * @return the first element in this list.
     * @throws    NoSuchElementException if this list is empty.
     */
    public E getFirst() {
	if (size==0)
	    throw new NoSuchElementException();

	return header.next.element;
    }

    /**
     * Returns the last element in this list.
     *
     * @return the last element in this list.
     * @throws    NoSuchElementException if this list is empty.
     */
    public E getLast()  {
	if (size==0)
	    throw new NoSuchElementException();

	return header.previous.element;
    }

    /**
     * Removes and returns the first element from this list.
     *
     * @return the first element from this list.
     * @throws    NoSuchElementException if this list is empty.
     */
    public E removeFirst() {
	return remove(header.next);
    }

    /**
     * Removes and returns the last element from this list.
     *
     * @return the last element from this list.
     * @throws    NoSuchElementException if this list is empty.
     */
    public E removeLast() {
	return remove(header.previous);
    }

    /**
     * Inserts the given element at the beginning of this list.
     *
     * @param o the element to be inserted at the beginning of this list.
     */
    public void addFirst(E o) {
	addBefore(o, header.next);
    }

    /**
     * Appends the given element to the end of this list.  (Identical in
     * function to the <tt>add</tt> method; included only for consistency.)
     *
     * @param o the element to be inserted at the end of this list.
     */
    public void addLast(E o) {
	addBefore(o, header);
    }

    /**
     * Returns <tt>true</tt> if this list contains the specified element.
     * More formally, returns <tt>true</tt> if and only if this list contains
     * at least one element <tt>e</tt> such that <tt>(o==null ? e==null
     * : o.equals(e))</tt>.
     *
     * @param o element whose presence in this list is to be tested.
     * @return <tt>true</tt> if this list contains the specified element.
     */
    public boolean contains(Object o) {
        return indexOf(o) != -1;
    }

    /**
     * Returns the number of elements in this list.
     *
     * @return the number of elements in this list.
     */
    public int size() {
	return size;
    }

    /**
     * Appends the specified element to the end of this list.
     *
     * @param o element to be appended to this list.
     * @return <tt>true</tt> (as per the general contract of
     * <tt>Collection.add</tt>).
     */
    public boolean add(E o) {
	addBefore(o, header);
        return true;
    }

    /**
     * Removes the first occurrence of the specified element in this list.  If
     * the list does not contain the element, it is unchanged.  More formally,
     * removes the element with the lowest index <tt>i</tt> such that
     * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt> (if such an
     * element exists).
     *
     * @param o element to be removed from this list, if present.
     * @return <tt>true</tt> if the list contained the specified element.
     */
    public boolean remove(Object o) {
        if (o==null) {
            for (Entry<E> e = header.next; e != header; e = e.next) {
                if (e.element==null) {
                    remove(e);
                    return true;
                }
            }
        } else {
            for (Entry<E> e = header.next; e != header; e = e.next) {
                if (o.equals(e.element)) {
                    remove(e);
                    return true;
                }
            }
        }
        return false;
    }

    /**
     * Appends all of the elements in the specified collection to the end of
     * this list, in the order that they are returned by the specified
     * collection's iterator.  The behavior of this operation is undefined if
     * the specified collection is modified while the operation is in
     * progress.  (This implies that the behavior of this call is undefined if
     * the specified Collection is this list, and this list is nonempty.)
     *
     * @param c the elements to be inserted into this list.
     * @return <tt>true</tt> if this list changed as a result of the call.
     * @throws NullPointerException if the specified collection is null.
     */
    public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
    }

    /**
     * Inserts all of the elements in the specified collection into this
     * list, starting at the specified position.  Shifts the element
     * currently at that position (if any) and any subsequent elements to
     * the right (increases their indices).  The new elements will appear
     * in the list in the order that they are returned by the
     * specified collection's iterator.
     *
     * @param index index at which to insert first element
     *		    from the specified collection.
     * @param c elements to be inserted into this list.
     * @return <tt>true</tt> if this list changed as a result of the call.
     * @throws IndexOutOfBoundsException if the specified index is out of
     *            range (<tt>index &lt; 0 || index &gt; size()</tt>).
     * @throws NullPointerException if the specified collection is null.
     */
    public boolean addAll(int index, Collection<? extends E> c) {
        if (index < 0 || index > size)
            throw new IndexOutOfBoundsException("Index: "+index+
                                                ", Size: "+size);
        Object[] a = c.toArray();
        int numNew = a.length;
        if (numNew==0)
            return false;
	modCount++;

        Entry<E> successor = (index==size ? header : entry(index));
        Entry<E> predecessor = successor.previous;
	for (int i=0; i<numNew; i++) {
            Entry<E> e = new Entry<E>((E)a[i], successor, predecessor);
            predecessor.next = e;
            predecessor = e;
        }
        successor.previous = predecessor;

        size += numNew;
        return true;
    }

    /**
     * Removes all of the elements from this list.
     */
    public void clear() {
        Entry<E> e = header.next;
        while (e != header) {
            Entry<E> next = e.next;
            e.next = e.previous = null;
            e.element = null;
            e = next;
        }
        header.next = header.previous = header;
        size = 0;
	modCount++;
    }


    // Positional Access Operations

    /**
     * Returns the element at the specified position in this list.
     *
     * @param index index of element to return.
     * @return the element at the specified position in this list.
     *
     * @throws IndexOutOfBoundsException if the specified index is out of
     * range (<tt>index &lt; 0 || index &gt;= size()</tt>).
     */
    public E get(int index) {
        return entry(index).element;
    }

    /**
     * Replaces the element at the specified position in this list with the
     * specified element.
     *
     * @param index index of element to replace.
     * @param element element to be stored at the specified position.
     * @return the element previously at the specified position.
     * @throws IndexOutOfBoundsException if the specified index is out of
     *		  range (<tt>index &lt; 0 || index &gt;= size()</tt>).
     */
    public E set(int index, E element) {
        Entry<E> e = entry(index);
        E oldVal = e.element;
        e.element = element;
        return oldVal;
    }

    /**
     * Inserts the specified element at the specified position in this list.
     * Shifts the element currently at that position (if any) and any
     * subsequent elements to the right (adds one to their indices).
     *
     * @param index index at which the specified element is to be inserted.
     * @param element element to be inserted.
     *
     * @throws IndexOutOfBoundsException if the specified index is out of
     *		  range (<tt>index &lt; 0 || index &gt; size()</tt>).
     */
    public void add(int index, E element) {
        addBefore(element, (index==size ? header : entry(index)));
    }

    /**
     * Removes the element at the specified position in this list.  Shifts any
     * subsequent elements to the left (subtracts one from their indices).
     * Returns the element that was removed from the list.
     *
     * @param index the index of the element to removed.
     * @return the element previously at the specified position.
     *
     * @throws IndexOutOfBoundsException if the specified index is out of
     * 		  range (<tt>index &lt; 0 || index &gt;= size()</tt>).
     */
    public E remove(int index) {
        return remove(entry(index));
    }

    /**
     * Return the indexed entry.
     */
    private Entry<E> entry(int index) {
        if (index < 0 || index >= size)
            throw new IndexOutOfBoundsException("Index: "+index+
                                                ", Size: "+size);
        Entry<E> e = header;
        if (index < (size >> 1)) {
            for (int i = 0; i <= index; i++)
                e = e.next;
        } else {
            for (int i = size; i > index; i--)
                e = e.previous;
        }
        return e;
    }


    // Search Operations

    /**
     * Returns the index in this list of the first occurrence of the
     * specified element, or -1 if the List does not contain this
     * element.  More formally, returns the lowest index i such that
     * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>, or -1 if
     * there is no such index.
     *
     * @param o element to search for.
     * @return the index in this list of the first occurrence of the
     * 	       specified element, or -1 if the list does not contain this
     * 	       element.
     */
    public int indexOf(Object o) {
        int index = 0;
        if (o==null) {
            for (Entry e = header.next; e != header; e = e.next) {
                if (e.element==null)
                    return index;
                index++;
            }
        } else {
            for (Entry e = header.next; e != header; e = e.next) {
                if (o.equals(e.element))
                    return index;
                index++;
            }
        }
        return -1;
    }

    /**
     * Returns the index in this list of the last occurrence of the
     * specified element, or -1 if the list does not contain this
     * element.  More formally, returns the highest index i such that
     * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>, or -1 if
     * there is no such index.
     *
     * @param o element to search for.
     * @return the index in this list of the last occurrence of the
     * 	       specified element, or -1 if the list does not contain this
     * 	       element.
     */
    public int lastIndexOf(Object o) {
        int index = size;
        if (o==null) {
            for (Entry e = header.previous; e != header; e = e.previous) {
                index--;
                if (e.element==null)
                    return index;
            }
        } else {
            for (Entry e = header.previous; e != header; e = e.previous) {
                index--;
                if (o.equals(e.element))
                    return index;
            }
        }
        return -1;
    }

    // Queue operations.

    /**
     * Retrieves, but does not remove, the head (first element) of this list.
     * @return the head of this queue, or <tt>null</tt> if this queue is empty.
     * @since 1.5
     */
    public E peek() {
        if (size==0)
            return null;
        return getFirst();
    }

    /**
     * Retrieves, but does not remove, the head (first element) of this list.
     * @return the head of this queue.
     * @throws NoSuchElementException if this queue is empty.
     * @since 1.5
     */
    public E element() {
        return getFirst();
    }

    /**
     * Retrieves and removes the head (first element) of this list.
     * @return the head of this queue, or <tt>null</tt> if this queue is empty.
     * @since 1.5
     */
    public E poll() {
        if (size==0)
            return null;
        return removeFirst();
    }

    /**
     * Retrieves and removes the head (first element) of this list.
     * @return the head of this queue.
     * @throws NoSuchElementException if this queue is empty.
     * @since 1.5
     */
    public E remove() {
        return removeFirst();
    }

    /**
     * Adds the specified element as the tail (last element) of this list.
     *
     * @param o the element to add.
     * @return <tt>true</tt> (as per the general contract of
     * <tt>Queue.offer</tt>)
     * @since 1.5
     */
    public boolean offer(E o) {
        return add(o);
    }

    /**
     * Returns a list-iterator of the elements in this list (in proper
     * sequence), starting at the specified position in the list.
     * Obeys the general contract of <tt>List.listIterator(int)</tt>.<p>
     *
     * The list-iterator is <i>fail-fast</i>: if the list is structurally
     * modified at any time after the Iterator is created, in any way except
     * through the list-iterator's own <tt>remove</tt> or <tt>add</tt>
     * methods, the list-iterator will throw a
     * <tt>ConcurrentModificationException</tt>.  Thus, in the face of
     * concurrent modification, the iterator fails quickly and cleanly, rather
     * than risking arbitrary, non-deterministic behavior at an undetermined
     * time in the future.
     *
     * @param index index of first element to be returned from the
     *		    list-iterator (by a call to <tt>next</tt>).
     * @return a ListIterator of the elements in this list (in proper
     * 	       sequence), starting at the specified position in the list.
     * @throws    IndexOutOfBoundsException if index is out of range
     *		  (<tt>index &lt; 0 || index &gt; size()</tt>).
     * @see List#listIterator(int)
     */
    public ListIterator<E> listIterator(int index) {
	return new ListItr(index);
    }

    private class ListItr implements ListIterator<E> {
	private Entry<E> lastReturned = header;
	private Entry<E> next;
	private int nextIndex;
	private int expectedModCount = modCount;

	ListItr(int index) {
	    if (index < 0 || index > size)
		throw new IndexOutOfBoundsException("Index: "+index+
						    ", Size: "+size);
	    if (index < (size >> 1)) {
		next = header.next;
		for (nextIndex=0; nextIndex<index; nextIndex++)
		    next = next.next;
	    } else {
		next = header;
		for (nextIndex=size; nextIndex>index; nextIndex--)
		    next = next.previous;
	    }
	}

	public boolean hasNext() {
	    return nextIndex != size;
	}

	public E next() {
	    checkForComodification();
	    if (nextIndex == size)
		throw new NoSuchElementException();

	    lastReturned = next;
	    next = next.next;
	    nextIndex++;
	    return lastReturned.element;
	}

	public boolean hasPrevious() {
	    return nextIndex != 0;
	}

	public E previous() {
	    if (nextIndex == 0)
		throw new NoSuchElementException();

	    lastReturned = next = next.previous;
	    nextIndex--;
	    checkForComodification();
	    return lastReturned.element;
	}

	public int nextIndex() {
	    return nextIndex;
	}

	public int previousIndex() {
	    return nextIndex-1;
	}

	public void remove() {
            checkForComodification();
            Entry<E> lastNext = lastReturned.next;
            try {
                LinkedList.this.remove(lastReturned);
            } catch (NoSuchElementException e) {
                throw new IllegalStateException();
            }
	    if (next==lastReturned)
                next = lastNext;
            else
		nextIndex--;
	    lastReturned = header;
	    expectedModCount++;
	}

	public void set(E o) {
	    if (lastReturned == header)
		throw new IllegalStateException();
	    checkForComodification();
	    lastReturned.element = o;
	}

	public void add(E o) {
	    checkForComodification();
	    lastReturned = header;
	    addBefore(o, next);
	    nextIndex++;
	    expectedModCount++;
	}

	final void checkForComodification() {
	    if (modCount != expectedModCount)
		throw new ConcurrentModificationException();
	}
    }

    private static class Entry<E> {
	E element;
	Entry<E> next;
	Entry<E> previous;

	Entry(E element, Entry<E> next, Entry<E> previous) {
	    this.element = element;
	    this.next = next;
	    this.previous = previous;
	}
    }

    private Entry<E> addBefore(E o, Entry<E> e) {
	Entry<E> newEntry = new Entry<E>(o, e, e.previous);
	newEntry.previous.next = newEntry;
	newEntry.next.previous = newEntry;
	size++;
	modCount++;
	return newEntry;
    }

    private E remove(Entry<E> e) {
	if (e == header)
	    throw new NoSuchElementException();

        E result = e.element;
	e.previous.next = e.next;
	e.next.previous = e.previous;
        e.next = e.previous = null;
        e.element = null;
	size--;
	modCount++;
        return result;
    }

    /**
     * Returns a shallow copy of this <tt>LinkedList</tt>. (The elements
     * themselves are not cloned.)
     *
     * @return a shallow copy of this <tt>LinkedList</tt> instance.
     */
//    public Object clone() {
//        LinkedList<E> clone = null;
//	try {
//	    clone = (LinkedList<E>) super.clone();
//	} catch (Exception e) {
//	    throw new InternalError();
//	}
//
//        // Put clone into "virgin" state
//        clone.header = new Entry<E>(null, null, null);
//        clone.header.next = clone.header.previous = clone.header;
//        clone.size = 0;
//        clone.modCount = 0;
//
//        // Initialize clone with our elements
//        for (Entry<E> e = header.next; e != header; e = e.next)
//            clone.add(e.element);
//
//        return clone;
//    }

    /**
     * Returns an array containing all of the elements in this list
     * in the correct order.
     *
     * @return an array containing all of the elements in this list
     * 	       in the correct order.
     */
    public Object[] toArray() {
	Object[] result = new Object[size];
        int i = 0;
        for (Entry<E> e = header.next; e != header; e = e.next)
            result[i++] = e.element;
	return result;
    }

    /**
     * Returns an array containing all of the elements in this list in
     * the correct order; the runtime type of the returned array is that of
     * the specified array.  If the list fits in the specified array, it
     * is returned therein.  Otherwise, a new array is allocated with the
     * runtime type of the specified array and the size of this list.<p>
     *
     * If the list fits in the specified array with room to spare
     * (i.e., the array has more elements than the list),
     * the element in the array immediately following the end of the
     * collection is set to null.  This is useful in determining the length
     * of the list <i>only</i> if the caller knows that the list
     * does not contain any null elements.
     *
     * @param a the array into which the elements of the list are to
     *		be stored, if it is big enough; otherwise, a new array of the
     * 		same runtime type is allocated for this purpose.
     * @return an array containing the elements of the list.
     * @throws ArrayStoreException if the runtime type of a is not a
     *         supertype of the runtime type of every element in this list.
     * @throws NullPointerException if the specified array is null.
     */
    public <T> T[] toArray(T[] a) {
        if (a.length < size)
            a = (T[])java.lang.reflect.Array.newInstance(
                                a.getClass().getComponentType(), size);
        int i = 0;
	Object[] result = a;
        for (Entry<E> e = header.next; e != header; e = e.next)
            result[i++] = e.element;

        if (a.length > size)
            a[size] = null;

        return a;
    }

    private static final long serialVersionUID = 876323262645176354L;

    /**
     * Save the state of this <tt>LinkedList</tt> instance to a stream (that
     * is, serialize it).
     *
     * @serialData The size of the list (the number of elements it
     *		   contains) is emitted (int), followed by all of its
     * elements (each an Object) in the proper order.
     */
//    private void writeObject(java.io.ObjectOutputStream s)
//        throws java.io.IOException {
//	// Write out any hidden serialization magic
//	s.defaultWriteObject();
//
//        // Write out size
//        s.writeInt(size);
//
//	// Write out all elements in the proper order.
//        for (Entry e = header.next; e != header; e = e.next)
//            s.writeObject(e.element);
//    }

    /**
     * Reconstitute this <tt>LinkedList</tt> instance from a stream (that is
     * deserialize it).
     */
//    private void readObject(java.io.ObjectInputStream s)
//        throws java.io.IOException, ClassNotFoundException {
//	// Read in any hidden serialization magic
//	s.defaultReadObject();
//
//        // Read in size
//        int size = s.readInt();
//
//        // Initialize header
//        header = new Entry<E>(null, null, null);
//        header.next = header.previous = header;
//
//	// Read in all elements in the proper order.
//	for (int i=0; i<size; i++)
//            addBefore((E)s.readObject(), header);
//    }
}
